1. Field of the Invention
This invention relates to a combined (or multi-piece) oil ring feasible for oil control, fitted to a piston of an internal combustion engine or compressor.
2. Description of the Prior Art
In recent years, internal combustion engines have increasingly higher performances, and those of a higher speed and a higher load have been developed. Accordingly, sliding members such as cylinders and piston rings are improved in variety so that they Can be improved in their heat resistance, wear resistance and seizure resistance. Nowadays, in addition to these properties, it is desirable to decrease fuel consumption and lubricating oil consumption. For this purpose, it has become popular, particularly in gasoline engines, to use three-piece combined oil rings.
Three-piece combined oil rings are comprised of upper and lower two side rails and a spacer expander provided between them, and have the following advantages: The upper and lower rails are set separate, and hence they can readily follow any minute deformation of a cylinder bore. The upper and lower rails come into contact with both the top wall and the bottom wall of an oil ring groove, and hence they can promise an excellent sealing effect. The upper and lower rails can be assembled in such a way that their gaps (rail-end gaps) can be set aside each other by 180.degree., and hence the rings by no means open at their gaps. As a result, the lubricating oil consumption can be decreased.
FIG. 9 illustrates an example of combined oil ring of this type. This combined oil ring 11 is comprised of upper and lower circular side rails 13 and 15 and a spacer expander 17 U-shaped in its cross section and fitted between the side rails 13 and 15 in a compressed stated. The spacer expander 17 is provided with cutouts 19 so formed as to play a role of guides through which oil is led to the oil ring groove.
It is prevailing that the side rails 13 and 15 have rigid layers 21 formed on their whole or some surfaces. For example, Japanese Laid-open Patent Application No. 58-136771 discloses that side rails are provided with nitrided layers on their whole surfaces. Japanese Laid-open Utility Model Application No. 53-147309 discloses that side rails are provided with hardened layers formed on their inner and outer edge faces by hardening or nitriding.
The above conventional combined oil rings, however, have the following problems remaining unsolved.
In the first place, although the combined oil rings having upper and lower rails separately provided are advantageous in that they can readily follow any minute deformation of a cylinder bore, they often can not exhibit a satisfactory conformable property in engines comprised of a cylinder block in which a plurality of cylinders are integrally formed, because such engines have particularly a high temperature at the part between combustion chambers to cause a non-uniformity in temperature distribution, resulting in a loss of roundness of the cylinder bore because of a difference in thermal expansion.
Secondly, in the case where the whole surfaces of side rails are treated by nitriding as disclosed in Japanese Laid-open Patent Application No. 58-136771, there remain the problems that the side rails tend to embrittle to cause cracks and that the oil ring groove tends to be worn because of contact of the side rails with the piston oil ring groove.
Thirdly, in the case where the inner and outer edge faces are treated by nitriding as disclosed in Japanese Laid-open Utility Model Application No. 53-147309, the problem of embrittlement can be lessened compared with the case where the whole surfaces are treated by nitriding, but, because of the inner edge face made rigid, the problem of the wear of oil ring groove has not been settled. That is, inner edge faces of side rails and inside corners thereof at the gaps tend to come in touch with the top and bottom walls of the oil ring groove, so that the oil ring groove may be abraded.